This invention relates to the fabrication of a DRAM cell consisting of a MOS transistor and a capacitor which is stacked over the transistor, and more particularly to a method of producing a cylindrical storage node of the stacked capacitor.
In MOS DRAMs, since the introduction of the 1k DRAM in the early 1970s the memory density has been multiplied by 4 every 3 years with a reduction in memory cell area to 30-40%. Although the memory cell area is reduced, it is necessary to retain a sufficiently large storage capacitance in order to keep immunity to soft errors. In this regard, it is advantageous to employ stacked capacitor cells.
For further enhancement of the density of DRAM cells with furhter reduction in memory cell area, a stacked capacitor cell having a cylindrical capacitor is under development. The storage node of the cylindrical capacitor has a hollow cylinder of polysilicon (polycrystalline silicon) which stands vertically, and both the outer and inner surfaces of the cylinder are used as a capacitance area. With this structure the storage capacitance can be increased without enlarging the memory cell area.
For example, JP-A 2-260454 (1990) shows a method of producing the above-mentioned cylindrical capacitor.
In the accompanying drawings, FIG. 4 shows the structure of a memory cell produced by using a method according to JP-A 2-260454. On a p-type silicon substrate 60, a field oxide film 62 defines an active area. The memory cell has a MOS transistor 64 with a gate oxide film 66 and n.sup.+ -type source and drain regions 70, 72 in the substrate 60 and a gate electrode 68 on the substrate. Dielectric films 74 and 76 constitute an interlayer insulator. Bit line interconnection 82 on the interlayer insulator is connected to the source/drain region 70 of the transistor by a contact 80 provided by using a contact hole 78 in the insulator. The interconnection 82 is covered by a silicon oxide film 84, which is overlaid with a planarizing film 86 of borophosphosilicate glass (BPSG), viz. silicon oxide doped with boron and phosphorus. On the planarizing film 86 there is another silicon oxide film 88 which is used as an etch stop film. The films 84, 86 and 88 constitute a second interlayer insulator. A cylindrical capacitor 94 stands on the silicon oxide film 88. The capacitor consists of a cylindrical storage node (viz. a lower plate) 96 having a closed bottom, a dielectric film 98 which covers the outer and inner surfaces of the storage node 96 and a conductor film (upper plate) 100 deposited on the dielectric film 98. The bottom of the storage node 96 is connected to one of the source/drain regions 70, 72 of the transistor by a contact 92 which is provided by using a contact hole 90 in the interlayer insulators.
FIGS. 5(A) to 5(F) illustrate a known process of forming the cylindrical storage node 96 in FIG. 4.
Referring to FIG. 5(A), the surface of the BPSG film 86 is planarized by a reflow treatment, and the silicon oxide film 88 is deposited to a thickness sufficient for an etch stop film. Then the contact hole 90 for the storage node is formed by photolithography and etching.
Referring to FIG. 5(B), a conductor film (not shown) is deposited on the etch stop film 88 until the contact hole 90 is filled with the deposited conductor 92, and then the conductor film is removed by an etch-back treatment so as to leave the conductor 92 only in the contact hole 90. It is inevitable that the etch stop film 88 also undergoes etching and consequently) considerably reduces thickness.
Referring to FIG. 5(C), a polysilicon film 96A is deposited on the etch stop film 88, and a relatively thick BPSG film 102 is deposited on the polysilicon film 96A. Then, the BPSG film 102 and the underlying polysilicon film 96A are patterned into the shape of a solid cylinder by photolithography and etching. The BPSG film 102 in the cylindrical shape becomes a temporary core of the aimed cylindrical storage node.
Referring to FIG. 5(D), a polysilicon film 96B is deposited on the etch stop film 88 so as to entirely cover the BPSG core 102 and the underlying polysilicon film 96A.
Referring to FIG. 5(E), the polysilicon film 96B is subjected to etch-back by anisotropic etching so as to leave the film 96B only as a cylindrical sidewall on the side face of the BPSG core 102 and the underlying polysilicon film 96A. After that, as shown in FIG. 5(F), the BPSG core 102 is completely removed by selective etching with high selectivity to the BPSG core 102 relative to the etch stop film 88. The polysilicon film 96A lying on the etch stop film 88 and the cylindrical sidewall part of the polysilicon film 96B constitute the cylindrical storage node 96 shown in FIG. 4.
In the above process, the etch stop film 88 is repeatedly exposed to etching solutions or gases. Therefore, it is likely that the etch stop film 88 in FIG. 5(F) is unduly thin or has openings. Then, the etching solution penetrates into the BPSG film 86 and locally dissolves the film 86 to form some cavities 106. When cavities exist in the BPSG film 86, subsequent heat treatments will induce unexpected stress in the film 86, and the dielectric film of the capacitor may be damaged by the influence of the stress.
If the initial thickness of the etch stop film 88 is greatly increased to compensate for the inevitable loss of film thickness, it becomes difficult to form the contact hole 90 because etching is liable to terminate before reaching the substrate surface. Incompleteness of the contact hole 90 is a cause of an open contact defect of the storage capacitor and a resultant bit error.
When the etch stop film 88 is very thick, it is conceivable to omit the planarizing BPSG film 86 to avoid undesriable increase in the total thickness of interlayer insulators. However, in that case the etch stop film 88 has a stepped profile reflecting steps on the underlying layer. So, in the photolithography for forming the contact hole 90 a resist pattern is formed on an unplanarized surface, and therefore there is a possibility of inaccurate opening of the contact hole 90. For this reason, the storage node contact is likely to be defective.